Clean up default constructors/destructors

src/ComputingTasks.cc

src/ComputingTasks.hh

src/DataTree.cc

src/DataTree.hh

src/ExprNode.cc

src/ExprNode.hh

src/ExternalFunctionsTable.cc

src/ExternalFunctionsTable.hh

src/Statement.cc

src/Statement.hh

src/WarningConsolidation.hh

src/macro/MacroValue.hh